This invention relates to a solid state area imaging apparatus which convert a two dimensional optical image into an electrical signal.
An ordinary area imaging apparatus constructed two-dimensionally cannot avoid having a decrease in its photodiode area, which means a decrease in its sensitivity, as its packing density increases. Thus, it has been desired to three-dimensionally construct an area imaging apparatus with very large scale integration.
A hitherto known example of the three-dimensional structure is a CCD such as that shown in FIG. 1. It has transfer electrodes 1 and 2 on both sides of the semiconductor substrate 3, in which a charge is not only horizontally but also vertically transferred. The vertical transfer process therein is as follows.
Charges 4 are retained and transferred by a depletion region 5 in a front surface area of the substrate 3 and then injected into the inside of the substrate 3 by removing the depletion region. The injected charges are collected at the depletion region 6 formed in the back surface area of the substrate 3.
Such a vertical charge transfer has had a disadvantage in that the charge is laterally diffused after the injection, which decreases the charge collection efficiency on the other side of the substrate. A further disadvantage is the random noise caused by the fluctuation of the charge injection. It has also been necessary to drive the transfer electrodes with a high voltage so as to sufficiently extend the depletion region.
In order to get over said disadvantages, it is necessary to make the substrate thin, say, a few tens of microns. However, this makes the fabrication process very difficult when the chip size is large and also makes the substrate very fragile.